Security device

ABSTRACT

A security device comprises a guide (21) provided with two polarizing screens (22) between which are placed a plurality of stacked, movable elements. Each element contains means for causing the plane of polarization of incident light passing therethrough to be rotated, the amount of rotation depending on the position of the means for rotating relative to the other elements. Setting the stacked elements to various positions results in a particular pattern of colors when light is shown through the device. This pattern can be used as an output indicator to control access to an apparatus.

In a conventional key-operated lock, the key is shaped to move aplurality of tumblers to a selected combination of positions whichuniquely allows the lock to turn. In a combination lock, the tumblersare moved to different combinations of positions by means other than akey, but only one combination of tumbler positions will allow the lockto open. In a conventional combination lock, the lock must be virtuallydismantled and reassembled if the selected combination which allows thelock to turn is to be changed. When a security device which has aplurality of variable elements is used in combination with a computer,it is a relatively easy matter to adjust the selected combination ofstates which allows the lock to open, for example by feeding into thecomputer new instructions to allow the lock to open in response to a newcombination of element states.

According to one aspect of the invention, there is provided a method ofcontrolling access to apparatus comprising generating a random number,feeding input data into a security device in response to said randomnumber, feeding output data from said security device, generated inresponse to said input data, to a control device, generating referenceinformation in response to said random number generated and allowingaccess to said apparatus when the output data corresponds to saidreference data.

In one arrangement, the security device comprises a plurality ofrelatively movable elements which can be moved to any of a plurality ofcombinations of positions, said input data comprising instructions tomove respective elements to respective positions, each said elementvarying the plane of polarization of light passing through it accordingto its position, said output data being dependent on the colour of lightemerging through all of said variable elements and a pair of polarizingscreens.

According to another aspect of the invention there is provided asecurity device comprising a plurality of elements, each movable to anyof a plurality of positions and each causing rotation of the plane ofpolarization of light passing through it by an amount depending on itsposition, the device comprising a pair of plane polarizing screensmounted one on either side of the plurality of elements, such that thecolour of light having passed through the screens and elements dependson the positions of the elements relative to the screens. By moving theelements to a different combination of positions, different colours canbe generated. If the elements have different thicknesses at differentzones across the width of the screen, then the colours of the lightemerging from different zones will also be different. By feeding inputdata into the security device in the form of instructions as to theposition of each element, output data can be derived in the form of thecolour or a combination of colours of the light emerging from the screenwhen white light is shone upon the device.

An example of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of an element of an encoding device, and

FIG. 2 is a perspective view of an encoding device with four suchelements.

The element of FIG. 1 comprises a backing strip 11 of uniform crosssection formed of toughened polyvinyl chloride (pvc) with a handle 12 atone end bearing a label 13 identifying the element. A plurality of equalsized zones 14 are located along the strip. Each zone may be providedwith a covering layer which rotates the plane of polarization of lightpassing through it by an angle depending upon its thickness. Somecoatings have a preferred plane of polarization so that the lightpassing through it is affected not only by the thickness of the coatingbut also by its preferred plane of polarization. The Figure shows eightzones 14, each with a coating of polyethylene whose thickness has aselected constant value between nil and a maximum value.

FIG. 2 shows four such elements mounted in a linear guide 21 formedbetween two polarized screens 22 whose planes of polarization are inthis arrangement mutually at right angles. Each screen extends in thisarrangement across four zones on the elements. In this arrangement, thetop element is in position 2, with one zone extending beyond the righthand end of the guide 21, the next element in position 3, the nextelement in position 4 and the bottom element in position 2. When whitelight is shown through the screens with the four elements in thiscombination of positions, the light emerging at each zone will have acolour which depends on the thicknesses of the coatings on the fourelements aligned at the zone and the directions of the preferred planesof polarization of the coatings and the directions of the planes ofpolarisation of the screens. The four colours can be used as outputinformation for the security device, to be read in any desired way.

The guide 21 need not be linear. The elements could for example becircular with sector shaped zones, the guide allowing relative rotationof the elements.

The device can be used as follows: A computer generates a number in arandom fashion. The number is used to display to a user the positions(1-5) to which he should move each of the four elements on his securitydevice. At the same time, the computer will generate from the samenumber reference data whose use will be explained later. Having set thesecurity device to the positions instructed by the computer display, theuser will see four colours across the screen when he views the securitydevice against white light. He will key into the computer these fourcolours which the computer will compare with the reference data. If thecomparison is correct, the computer will enable access to apparatus. Ifthe user is attempting to use an incorrect security device, the colourshe views will be different, and on performing the comparison, thecomputer will register an error in the comparison and will not enableaccess to the apparatus. Several attempts may be allowed by thecomputer, after which it shuts down.

The actions of the operator could be dispensed with if the device isplaced in a reader operated by the computer. The reader moves theelements of the device in accordance with instructions derived from therandom number and then reads (for example by colour sensitivephotocells) the colours generated by the device in order to perform thecomparison.

Since it is difficult to analyse the polarization rotating properties ofthe elements of the security device from their physical measurements,the described security device is difficult to copy, so that unauthorisedaccess to the apparatus is difficult to achieve.

Thus the invention prevents an unauthorised user, or anyone else forthat matter, from writing down the colour combination required tooperate the apparatus. Therefore, even if someone has worked out thealgorithm on which the computer functions, the colour code cannot beascertained, so again preventing operation of the apparatus.

I claim:
 1. A security device comprising:a plurality of stackedelements, each movable to any of a plurality of positions relative tothe others; means carried by each element for causing rotation of theplane of polarization of incident light passing therethrough by anamount depending on its position relative to the other elements; and apair of plane polarizing screens mounted one on either side of the stackof elements such that the color of light having passed through thescreen and elements depends on the positions of the elements relative tothe screens and relative to each other.
 2. A device as claimed in claim1 wherein each element comprises a plurality of zones, each causingrotation of the plane of polarization of light passing therethrough by apredetermined amount.
 3. A device as claimed in claim 2 wherein saidscreens extend across a plurality of said zones.
 4. A device as claimedin claim 2 wherein each zone has a predetermined thickness.
 5. A deviceas claimed in claim 1 wherein each element comprises a backing strip anda coating.